1. Field of the Invention
The present invention relates to a gallium nitride (GaN) based semiconductor laser and an image exposure device. More particularly, the present invention relates to a GaN based semiconductor laser, which is used as a light source in an exposure device for exposing a silver halide photosensitive material, and an image exposure device using the GaN based semiconductor laser as a light source.
2. Description of the Related Art
Conventionally used semiconductor lasers include a gallium arsenic (GaAs) based semiconductor laser, which is formed by laminating semiconductor compounds, such as AlGaInP, AlGaAs, or InGaAsP, on a GaAs substrate. In the GaAs based semiconductor laser, in which GaAs is used in a substrate, either InGaAs or GaAs, used in a counter electrode, absorbs light within a wavelength range including an emission wavelength of the semiconductor laser. Therefore, the substrate or the counter electrode absorbs stray light resulting from electroluminescense (EL light). Further, when a stripe geometry has been employed, where emission light is confined within a narrow region which typically has a width of the order of several microns of an active layer, stray light out of the stripe region has rarely been a problem.
In recent years, an exposure apparatus for exposing a photosensitive material using a semiconductor laser has been utilized in various applications, one such application being a laser printer. In order to improve the quality of an exposed image, it is effective to shorten a wavelength of laser light to make the spot diameter of the laser beam as small as possible.
Development of a semiconductor laser with a shorter oscillation wavelength (hereinafter referred to as a “shorter wavelength laser,”) such as a blue-violet semiconductor laser, is steadily progressing, and a GaN based semiconductor laser is approaching the realm of practical use. Since a sapphire substrate or a silicon carbide (SiC) substrate used in the GaN based semiconductor laser is transparent to light of an emission wavelength of the semiconductor laser, EL light is emitted in random directions. Further, stray light is reflected on the surface of the semiconductor chip and either returns to areas in the vicinity of the active region or is reflected several times to generate stray light of various patterns.
This unintended stray light causes a problem that, when a GaN based semiconductor laser is used as a light source of an exposing apparatus, it fails to provide a sharp-edged laser beam spot even if a small diameter of the beam is accomplished through manipulation of an optical system. Particularly, in a silver halide system, in which a highly sensitive silver halide photosensitive material is exposed to form a continuous tone high-quality photographic image, the ambiguous spot edge problem is critical, comparing with an electrophotography process, in which an image is formed of halftone dots.
For example, as shown in FIGS. 13A and 13B, when light output from a GaN based semiconductor laser 1 is condensed onto a surface of a sheet of photographic paper 3 through a lens 2, the laser light 4, which is laser-oscillated at a predetermined wavelength, forms a spot 5 having a predetermined size, accompanied by a pattern 7, which has an ambiguous edge due to EL light 6 having random emitting positions, random directions and random wavelengths. While the power of the order of 0.1 mW is sufficient for exposing the highly sensitive silver halide photosensitive material, intensity of the EL light on the sheet of photographic paper is significantly high, as shown in FIG. 14, even in the low-exposure-intensity range where the light intensity (light output) of the laser spot on the sheet of the photographic paper is about 0.1 mW. Therefore, for example, when a stripe pattern shown in FIG. 15A, in which each line formed has a width being the same as the diameter of the spot 5, areas between the lines are exposed by the EL light 6 and this results in a less sharp image, as shown in FIG. 15B.